Polarizers used in polarizing plates, optical devices enabling natural light or optional polarized light to be converted into polarized light in a certain direction, have been widely used in display devices such as liquid crystal displays and organic light emitting diodes. As a polarizer currently used in such display devices, a polyvinyl alcohol-based polarizing film containing an iodine compound or a dichroic dye and having molecular chains oriented in a predetermined direction has generally been used.
The polyvinyl alcohol-based polarizing film may be prepared by dyeing a polyvinyl alcohol film with iodine or a dichroic dye, and then stretching the film in a predetermined direction and crosslinking the film. In the case, the stretching process may be performed through wet stretching undertaken in a solution such as an aqueous boric acid solution or an aqueous iodine solution, dry stretching undertaken in the atmosphere, or the like, and a stretching magnification of 5 or more times may be generally provided. However, in such a preparing process according to the related art, a polyvinyl alcohol-based film needs to have a thickness exceeding 60 μm, prior to a stretching thereof, in order to perform stretching thereon without the occurrence of breakage. In a case in which the thickness of the polyvinyl alcohol-based film prior to the stretching is less than 60 μm, a degree of swelling in the polyvinyl alcohol-based film may be increased, and the modulus acting per unit area during the stretching process may be increased due to the small thickness, such that breakage of the film may easily occur.
Meanwhile, in accordance with the recent trend for the thinning of display devices, polarizing plates also need to have a reduced thickness. However, in a case in which a polyvinyl alcohol-based film having a thickness exceeding 60 μm is used according to the related art, reducing the thickness of a polarizer may be restricted. Therefore, research into manufacturing a polarizer having a reduced thickness has been carried out.
Korean Patent Laid-Open Publication No. 2010-0071998 discloses a method for manufacturing a thin polarizer using a laminate fabricated by coating a hydrophilic polymer layer on a base layer, or co-extruding a base layer-forming material and a hydrophilic polymer layer-forming material. However, in the case of a coating or co-extruding method, a polyvinyl alcohol layer and the base layer may not be easily separated after stretching, and in order to perform the separation thereof, a high degree of peeling strength may be required. Thus, defects such as damage to or a deformation in the polyvinyl alcohol layer may be easily generated in a separation process and consequently, optical properties such as a degree of polarization and the like, of a polyvinyl alcohol film, are deteriorated. Moreover, in the case of using the coating or co-extruding method, since the polyvinyl alcohol film is manufactured by melting a polyvinyl alcohol resin and then extruding a film, or is manufactured by preparing the resin as a coating solution and then coating the solution, properties of the correspondingly manufactured polyvinyl alcohol film may be easily changed depending on extruding conditions, coating conditions or film forming conditions. Thus, properties of the finally manufactured polyvinyl alcohol film may be deteriorated and further, uniform properties may not be implemented therein.
Furthermore, Japanese Patent Laid-Open Publication No. 2012-118521 discloses a method for manufacturing a thin polarizer by bonding a polyvinyl alcohol film to one surface of a base film using a water-based polyvinyl alcohol adhesive and then stretching the film. However, in a case in which the polyvinyl alcohol film and the base film are bonded to each other using an adhesive as described above, a considerable amount of fine wrinkles in the form of cracks may occur in surfaces of the films due to a difference between degrees of swelling in the base film and the polyvinyl alcohol film. Further, curling may be intensified prior to stretching, leading to increased instability in the process, and, during a stretching process, breakage of the film may be easily generated. Moreover, in the case of performing stretching after the base film and the polyvinyl alcohol film are attached to each other with an adhesive, a curling phenomenon may occur in edges of the films after the stretching.